Even if it's not really "bullying" and just age-typical cattiness, that can have a profound effect on some kids. My experience is that girls that are around 7 are the worst. They are old enough to be manipulative but too young to have a good grasp for how their actions affect others and ruin relationships. They are old enough to realize how much power they have and they run wild with it. In third grade things have been MUCH better, although it could just be the class with a nice group of girls. You may want to talk more with your DD about the other kids and see if you can get a feel for whether they are upsetting her.